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J. Chloroplast (plant and algae only)

Diagram:

Transmission electron micrograph:

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Organelle found in plants and green algae, and which contains two membranes and a small genome. They are thought to originate from certain bacteria that have been assimilated by a primordial eukaryotic cell.

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Photosynthesis, that is the assimilation of inorganic carbon (CO2) into an organic form (sugar) using solar energy.

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Serves as a structural element and also influences their development and physiology.

Viruses

1. Animal Viruses

The human immunodeficiency virus (HIV)

2. Bacterial Viruses (Phages)

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Cell Physiology

1. Phagocytosis

Macrophages and poly-morpho-nuclear cells are white blood cells comprising an important part of the animal innate defense system. They destroy invading bacteria (or other pathogenic cells) by internalizing and digesting them chemically. They are therefore called ‘phagocytes‘, which means ‘cells that eat’ their prey.

Once the phagocyte tracks down the invading pathogen, it extends parts of its membrane towards the pathogen and engulfs it. The pathogen is then internalized into the phagocyte within a lysosome-like vesicle, which prevents it from harming the phagocyte. Shortly after, the pathogen is attacked by different chemicals produced by the phagocyte for exact purpose. For example, the phagocyte has enzymes that produce free oxygen radicals like superoxide and hypo-chlorite. These molecules are highly reactive and attack the pathogen’s membrane, DNA and proteins chemically. This leads to the destruction of the pathogen.

Phagocytosis of tuberculosis-causing bacteria (red rods) by a macrophage (source)

2. Apoptosis (programmed cell death)

There are some cases in nature, where cells are forced to commit suicide. This happens, for example, during embryonic development, where limbs form by creating spaces in the original tissue via the death of numerous cells. Apoptosis also occur when a virus-infested animal cell is attacked by a T-lymphocyte (a white blood cell that is part of the immune system). The T-cell perforates the membrane of the attacked cell and sends through the pore chemical messengers that re-program the attacked cell to commit suicide. This may be unfortunate to the attacked cell, but it saves the rest of the body by eliminating the virus along with cell it infested.

Apoptoses cells undergo multiple changes before they break down and phagocytosed by white blood cells. For example, their cytoskeleton breaks down, they become spherical, and vesicles form on their surface.